The dynamin family of proteins are found throughout the cell and potentially all are involved in membrane remodeling, fission or fusion. We have examined the conformational change of two family members, dynamin and DNM1. Previously, we have shown that dynamin and Dnm1 self-assemble into helical structure in the presence or absent of lipid. When GTP is added to the dynamin-lipid assemblies, the dynamin tubes constrict in diameter from 50 nm to 40 nm and Dnm1 tubes constrict from 100 nm to 50 nm. To further understand the mechanism of this dynamic process we examined the sample by time-resolved cryo-electron microscopy at different time points following GTP addition. The method allows the sample to be examined quickly in a thin layer of vitreous ice without the addition of stain or carbon support. We showed that immediately upon GTP addition dynamin constricts the underlying lipid bilayer in a concerted action and excess lipid bulges out at focal points along the constricted tubes. Following constriction, dynamin falls off the lipid bilayer, suggesting that dynamin-dynamin interactions are unstable in the constricted state. Our results demonstrate that dynamin can rapidly constrict the necks of coated pits and disassemble once its role in membrane fission is complete. Preliminary results suggest a similar behavior for Dnm1, however, the GTP-induced conformational change is significantly greater.